I picked up an old Onkyo HT-R380 amplifier that was stuck in standby / protect mode so pulled it apart to find out what the problem was.
This is a typical fault condition with a lot of old amplifiers as something fails and triggers one of many protection circuits that switch the unit off.
If you’re fixing one of these yourself it won’t work with the back panel removed as it forms a ground connection between the standby board and the main board.
I’ve produced a YouTube video that takes you through how this Onkyo powers up and what causes it to switch into protect or standby mode.
I traced the fault down to a Sanyo STK module that’s obsolete. I brought a presumably cloned one from China and it was more faulty then the original faulty one! Failure of the STK module is fairly major as it has three power amplifiers in it and it only needs one of the three to fail to shut the receiver down.
A faulty STK will probably put a high plus or minus DC voltage on the output which causes the shutdown. You could try removing the big white centre tapped resistor on the faulty channel. That would stop the amplifier going into protect mode.
If it were one of the main left or right channels that’s failed you could potentially swap the signals around on the input to the module to use one of the rear channels as the main left or right one. That’s getting a bit complicated though!
I’ve not tried these suggestions, it’s something you’d do at your own risk. It would be better to try and get a replacement STK module from a scrap unit. If you don’t know what you’re doing don’t even consider it.
Note: There’s a very high current +/- 50v on the circuit board and live mains on things like the transformer terminals. I sent myself flying across a room once by accidentally resting my arm on a live connection on a transformer. You could do a lot of damage to yourself or the amplifier if you don’t know what you’re doing.